Method of preparing porous carbonate apatite from natural bone

ABSTRACT

A method for preparing carbonate apatite from natural bones. The method includes obtaining cancellous bone particles; treating the bone particles with hot water and an organic solvent; repeating the treating step at least once; drying the bone particles; and heating the bone particles at 500° C. to 620° C. for 10 to 50 hours.

BACKGROUND

Current synthetic calcium-containing bone grafting materials usedclinically include calcium sulfate, calcium carbonate (coral-based), andvarious calcium phosphate compounds (e.g., tricalcium phosphate,hydroxyapatite). The synthetic calcium-containing materials have thedisadvantage of either resorbing too fast (e.g., calcium sulfate) or tooslow (e.g., hydroxyapatite), which would negatively impact bone growthand regeneration.

Carbonate apatite is the mineral structure of natural bone. Unlike thehighly crystalline structure of hydroxyapatite, carbonate apatite inbone has a lower degree of crystallinity. The lower degree ofcrystallinity allows the bone to turnover and remodel in vivo,particularly under the influence of mechanical stress conditions. If themineral can be isolated from natural bone without significantly changingits structure, it would be a more suitable bone grafting material.

Methods for preparing mineral from natural bone include those usingorganic solvents (e.g., ethylenediamine) under reflux conditions (see,e.g., U.S. Pat. Nos. 2,938,593, 5,167,961 and 5,417,975), and thoseusing heat treatment at a temperature generally higher than 900° C.(see, e.g., U.S. Pat. No. 4,654,464). These methods have variousdisadvantages, such as generating toxic solvent waste, altering thestructure of the bone mineral, and making bone mineral that causestissue reactions. See, e.g., Gardner, A. F., J Oral Surg Anesth HospDent Serv, 1964. 22: p. 332-40.

There is a need for a method that generates commercial quantity ofhighly porous, biocompatible and bioresorbable carbonate apatite fromnatural bone without significantly changing the structure of the mineralphase and that will not generate toxic waste.

SUMMARY

This invention is based on the unexpected discovery of a method capableof generating large quantity of porous carbonate apatite for variousmedical and dental surgical applications.

Accordingly, described herein is a method of preparing carbonate apatitefrom natural bone. The method includes obtaining cancellous boneparticles; treating the bone particles with hot water and an organicsolvent; repeating the treating step at least once; drying the boneparticles; and heating the bone particles at 500° C. to 620° C. for 10to 50 hours.

To obtain the bone particles, a cancellous bone can be first cleaned toremove adhering tissues, attached cartilages and cortical bone. The bonecan then be grounded into particles having a size ranging from about 2mm to about 15 mm, preferably from 5 mm to 10 mm. Uniformity of the sizeof the bone particles can facilitate the removal of loosely associatedorganic moieties from the bone (e.g., cells, cell debris and bloodcomponents). The hot water and organic solvent (e.g., ethanol andisopropanol) treatments serves to remove organic materials not directlyassociated with the bone (e.g., lipids, blood components, cells anddebris). The thusly prepared bone particles can then be heated at aspecific temperature range for a period of time (e.g., 10 to 50 hours).In some embodiments, the bone particles are heated at 500° C. to 620°C., preferably between 570° C. to 610° C., and more preferably between590° C. to 605° C.

The details of one or more embodiments of the invention are set forth inthe accompanying drawing and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawing, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing an X-ray diffraction pattern of sinteredanorganic bone mineral (hydroxyapatite) and anorganic carbonate apatite.

FIG. 2 is a graph showing an infrared spectra of anorganic carbonateapatite.

DETAILED DESCRIPTION

This invention relates to a method of preparing carbonate apatitemineral from natural bone of mammals that has a structure substantiallysimilar to the mineral of the intact bone. Particularly, the methodgenerates highly porous carbonate apatite from epiphysis of the mammals.Fresh epiphysis, the expanded head of bone containing mainly cancellous(spongy) bone tissue, is the main source of bone tissue harvested andprovided by suppliers. Since the spongy bone has pore sizes generally inthe range of 50 μm to 700 μm, it is the ideal structure for boneconduction and new bone growth.

The method described herein involves cleaning and processing of spongybone to remove organic components associated with bone, leaving theintact mineral crystal component (carbonate apatite) for bone conductionand growth applications. More specifically, the method includes heatingthoroughly cleaned bone particles under well controlled temperature andtime ranges that will not cause significant phase transition from lowcrystalline carbonate apatite structure to high crystallinehydroxyapatite.

Generally, the carbonate content of the carbonate apatite prepared fromthe method is in the range of about 1% to about 7% and the degree ofcrystallinity ranges from about 45% to about 65%. High crystallinehydroxyapatite has no carbonate and has a degree of crystallinitygenerally greater than 98%. Thus, carbonate apatite has smaller-sizedcrystals and less perfect crystal lattice than hydroxyapatite. Theclinical significance of carbonate apatite is that it has a structuresimilar to that of mineral in intact bone. Thus, when carbonate apatiteis implanted, it will behave more similarly to native bone mineral,i.e., allowing turnover or remodel in vivo upon new bone regeneration.

To practice the method, the cortical portion of bone, includingperiosteum, adhering soft tissues and attached cartilage tissues, isfirst removed and the spongy bone grounded into particles. The size ofthe particles is generally in the range of 2 mm to 15 mm, preferably inthe range of 5 mm to 10 mm. The ground bone particles are first washedwith cold water to remove some blood and marrow components associatedwith spongy bone. The washed bone particles then go through at least twocycles of hot water and organic solvent treatment. Washing with hotwater at boiling temperature for, e.g., 2 to 8 hours, is preferred,which can remove a good part of fat moieties, blood and cell debris fromthe bone. Organic alcoholic compounds can be used to remove lipids andlipoproteins from the bone. Washing with ethanol or isopropanol (fore.g., 16 to 24 hours at room temperature) is effective in this regard.Alcohol in combination with ether can be used in small scales, butalcohol alone is preferred to minimize toxic materials involved in themethod.

The cleaned bone particles are then dried (e.g., via air or oven) andheat-treated. The heating treatment can be carried out in a furnace. Forexample, the bone particles can be placed in crucibles (e.g., largecrucibles with about 50 g of particles in each), which are inserted intoa commercial furnace (e.g., Thermo Scientific). The amount of cleanedbone particles used can vary depending on the capacity of the furnace.Thus, the production can be scaled up by using larger capacity furnacesthat are commercially available.

The temperature of the furnace can be slowly raised to the targettemperature in the range of 500° C. to 620° C. within the first hour.Once the temperature has reached the target temperature, the boneparticles are heat-treated for a period of 10 to 50 hours, preferably at570° C. to 610° C. for 20 to 40 hours, and most preferably at 590° C. to605° C. for 22 to 33 hours. Generally, the duration of the heatingtreatment is related to the temperature range selected for thetreatment. For example, a higher temperature would require a shortertreatment. In any event, the time period selected should be one that issufficient to effectively remove organic materials from the boneparticles.

The thusly prepared carbonate apatite mineral has a structuresubstantially similar to the mineral in intact bone. The methoddescribed in this invention is applicable to all animal bone tissuesincluding but not limited to bovine, porcine, equine and ovine so longas the bones are cleaned and ground to the size as described.

The specific example below is to be construed as merely illustrative,and not limitative of the remainder of the disclosure in any waywhatsoever. Without further elaboration, it is believed that one skilledin the art can, based on the description herein, utilize the presentinvention to its fullest extent. All publications cited herein areherein incorporated by reference in their entirety.

Preparation of Natural Bone Mineral

Grounded bone particles received from supplier were first washed withcold water for 2-4 hours. The washed bone particles were then boiledwith water for 8 hours, the water changed every two hours. The hotwater-cleaned bone particles were extracted in isopropanol for 18 hoursto remove lipids and lipoproteins. The hot water- andisopropanol-cleaning steps were repeated once within the defined timeperiod (8 hours of hot water extraction and 18 hours of isopropanolextraction). The clean bone particles thus prepared were then air-driedor oven-dried for 24 hours.

After drying, approximate 50 g of the clean bone particles were placedin each of four crucibles. The crucibles were transferred into afurnace, and heat-treated at 595° C. for 33 hours. The crucibles werethen cooled in the furnace. Six furnaces were calibrated and usedsimultaneously to produce a large quantity of carbonate apatite,approximately 600 g.

Characterization of the Natural Bone Mineral

Carbonate apatite bone minerals produced by the above procedure werecharacterized by the following methods.

(1) X-Ray Diffraction

X-ray diffraction (XRD) pattern provides information about the latticestructure, the size of the crystals and the percent of crystallinity ofthe mineral. XRD analyses were conducted using PHILIPS PW1710 X-raydiffractometer, and scanned from 20 to 40 degrees (20 scale) to obtainkey reflections for the identification of apatite structure.

The XRD pattern of the anorganic bone mineral as prepared above showedtypical key mineral reflections at 211 and 002 of highly crystallinehydroxyapatite. See FIG. 1. The broad spectrum of the bone mineralthusly prepared indicated a smaller crystal size as compared to that ofsintered hydroxyapatite, which showed a characteristically narrowspectrum and sharper peaks. Also see FIG. 1. Using the Scherer equation,the average crystal size in the 002-direction was estimated to be 29.4nm for the bone mineral as prepared above, compared to 45.3 nm forhydroxyapatite. The percent crystallinity of mineral of the bone mineralwas determined to be 54.2±1.3% (based on an average of 3 lots) ascompared to 99% for hydroxyapatite.

(2) Infrared Spectroscopy

Infrared (IR) spectroscopy provides information relating to thestructure of a product in terms of its functional groups. The infraredspectra of the samples were obtained from samples prepared as KBrpellets with 10 weight % sample, and using a Fourier transform infraredspectrophotometer (Perkin-Elmer 983G).

The IR spectra of the mineral prepared above was similar to the spectraof natural bone consisting of carbonate apatite mineral including: thephosphate ion bands (sharp P-O υ4 antisymmetrical bending mode (550 cm⁻¹to 600 cm⁻¹); υ3 antisymmetrical stretching mode (1030 cm⁻¹ with 1100cm⁻¹ shoulder); and the carbonate ion (CO₃) band (υ2 antisymmetricalstretching mode, 1400 cm⁻¹-1500cm⁻¹). See FIG. 2.

The carbonate content in the bone mineral was determined based on astandard curve constructed by establishing the net integration area ofcarbonate absorption area (923-1332 cm⁻¹) to phosphate absorption area(1332-1633 cm⁻¹) verses different carbonate content. The bone mineral asprepared above had carbonate content of 2.0±0.3% (average of 3measurements ±S.D.).

(3) Ratio of Calcium to Phosphate

A sample of the mineral bone as prepared above were hydrolyzed in nitricacid to ensure total dissolution of the sample. After cooling, thesample was made up to volume, mixed and diluted for calcium andphosphate analyses by the Inductively Coupled Plasma Chromatographymethod. Standard solutions of calcium and phosphate were prepared forcalculation and correction. It was determined that the bone mineral hada calcium/phosphate ratio of 1.57 (average of 3 measurements ±S.D.).

(4) Non-Mineral Content (Residual Organic Content)

Residual protein content was determined by analyzing the % nitrogencontent in the mineral product by the combustion method. Result of theanalysis was recorded as weight of nitrogen found in the sample, and %nitrogen content was calculated. The residual protein content wascalculated based on the assumption that the average nitrogen content inprotein is 13.6% w/w (derived by dividing the average weight of nitrogenin all amino acids by the average molecular weight of all amino acids).In addition, the methanol extractable content was applied to determinethe lipid content in bone mineral product. It was determined that thebone mineral had a protein content of 0.75±0.08% and a methanolextractable of 0.03±0.01% (average of 6 measurements ±S.D.)

(5) In Vivo Study

An animal study was performed in a rabbit femoral defect model toevaluate the biocompatibility and efficacy of the derived naturalcarbonate apatite bone mineral prepared above. Nine animals wereimplanted with the bone mineral. They were sacrificed at 4, 8, and 14weeks. Histologically, the implanted mineral showed bone ingrowth asevidenced by new bone and bone marrow formation. There was no sign ofany safety issue, as there was a lack of inflammation and a low numberof giant cells associated with the implant mineral at all time points.It was concluded that the mineral product is biocompatible,osteoconductive without any significant unwanted tissue reaction.

OTHER EMBODIMENTS

All of the features disclosed in this specification may be combined inany combination. Each feature disclosed in this specification may bereplaced by an alternative feature serving the same, equivalent, orsimilar purpose. Thus, unless expressly stated otherwise, each featuredisclosed is only an example of a generic series of equivalent orsimilar features.

From the above description, one skilled in the art can easily ascertainthe essential characteristics of the present invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions. Thus, other embodiments are also within the claims.

What is claimed is:
 1. A method of preparing carbonate apatite from natural bone, the method consisting of: providing cancellous bone particles; treating the bone particles with hot water and an organic alcohol to remove lipids therefrom; repeating the treating step at least once; drying the treated bone particles; and heating the dried treated bone particles at 500° C. to 620° C. for 10 to 50 hours.
 2. The method of claim 1, wherein the size of the bone particles is 2 mm to 15 mm.
 3. The method of claim 2, wherein the size of the bone particles is 5 mm to 10 mm.
 4. The method of claim 1, wherein the bone particles are first treated with hot water, and then extracted with an organic alcohol at room temperature.
 5. The method of claim 1, wherein the organic alcohol is isopropanol.
 6. The method of claim 1, wherein the organic alcohol is ethanol.
 7. The method of claim 1, wherein the bone particles are treated with hot water for 2 to 8 hours.
 8. The method of claim 1, wherein the bone particles are treated with an organic alcohol for 16 to 24 hours.
 9. The method of claim 8, wherein the organic alcohol is isopropanol.
 10. The method of claim 1, wherein the temperature of the hot water is 80° C. to 100° C.
 11. The method of claim 1, wherein the bone particles are heated at 590° C. to 605° C.
 12. The method of claim 11, wherein the bone particles are heated for 22 to 33 hours.
 13. The method of claim 1, wherein the bone particles are heated for 20 to 40 hours.
 14. The method of claim 13, wherein the bone particles are heated for 22 to 33 hours.
 15. The method of claim 1, wherein the heating step is carried out in a furnace.
 16. A method of preparing carbonate apatite from natural bone, the method consisting of: obtaining cancellous bone particles by first removing adhering tissues, cartilage tissues and the cortical portion from a bone, and then grinding the cancellous portion of the bone into particles; treating the bone particles with hot water and an organic alcohol to remove lipids therefrom; repeating the treating step at least once; drying the treated bone particles; and heating the dried treated bone particles at 500° C. to 620° C. for 10 to 50 hours. 